Lead-free optical heavy flint glasses

ABSTRACT

The invention relates to lead-free optical glasses which have refractive indices n d  of between 1.65 and 1.80 and Abbe numbers ν d  of between 21 and 33 and possess the following composition (in % by weight, based on oxide) SiO 2  27-40; B 2 O 3  0-&lt;0.5; Al 2 O 3  0-6; Na 2 O 7-18; K 2 O 1-10; BaO 1-10; SrO 0-3; CaO 0.5-5; MgO 0-3; with BaO+SrO+CaO+MgO&lt;15; TiO 2  21-37; ZrO 2  0-7; Nb 2 O 5  5-17; WO 3  0.1-7.

[0001] The invention relates to lead-free optical glasses which haverefractive indices n_(d) of between 1.65 and 1.80 and Abbe numbers ν_(d)of between 21 and 33. These glasses belong to the optical glass typeconsisting of the heavy flint glasses (HF).

[0002] Since, in recent years, the glass components PbO and As₂O₃ havebeen considered to be environmentally polluting in public discussions,the manufacturers of optical apparatuses also require PbO-free andpreferably also As₂O₃-free glasses having the respective opticalproperties.

[0003] It is desirable to dispense with PbO also for the production oflight glass parts, i.e. of glasses having a low density.

[0004] It is as a rule not possible to reproduce the desired optical andtechnical glass properties influenced by PbO by simply replacing leadoxide by one or more components. Instead, new developments orwide-ranging changes in the glass composition are necessary.

[0005] The patent literature already includes some publications in whichlead-free glasses having optical values from said range are described.However, these glasses have a very wide range of disadvantages.

[0006] DE 32 164 51 A describes optical lightweight glasses having arefractive index n_(d) of >1.70 and an Abbe number ν_(d) of ≧22 and adensity ρ of ≦3.5 g/cm³. These glasses contain up to 3% by weight ofB₂O₃, a component which is aggressive towards Pt. If such glasses aremelted in Pt crucibles or if they come into contact with other Ptcomponents, which would improve the homogeneity and low bubble count ofthe glasses, they have a higher level of Pt impurities, with the resultthat their transmittance is adversely affected.

[0007] U.S. Pat. No. 3,589,918 describes an optical glass for lenssystems comprising the glass system Si0 ₂-K₂O-TiO₂-Sb₂O₃. The high Sb₂O₃contents of up to 45% by weight in this glass make the glass susceptibleto separation and heavy and adversely affect its transmittance so thatit is not suitable for modern applications in optics.

[0008] JP 53-16718 A discloses glasses having high contents of divalentoxides (MO=MgO+CaO+SrO+BaO+ZnO 15-50% by weight) and relatively lowcontents of TiO₂ (1-25% by weight). These glasses have Abbe numbers ofbetween 30 and 45. Owing to the high MO content, their stability tocrystallization is low.

[0009] JP 52-25812 A discloses TiO₂-and Nb₂O₅-glasses whose compositionsvary over a wide range. According to the examples, the glasses have veryhigh (25-45% by weight) or very low (5% by weight) of Nb₂O₅ contents.The same applies to the MO content (21 and 30% by weight and 0-5% byweight, respectively). These glasses having TiO₂ contents of up to 50%by weight are also not sufficiently stable to crystallization foreconomical continuous production.

[0010] It is an object of the invention to provide lead-free opticalglasses having a refractive index n_(d) of between 1.65 and 1.80 and anAbbe number ν_(d) of between 21 and 33, which possess good melting andprocessing properties and have good chemical resistance, good stabilityto crystallization and a low density.

[0011] This object is achieved by glasses described in Patent claim 1.

[0012] The good fusibility meltability of the glasses is achieved by thebalanced proportions of fluxes (Na₂O, K₂O) to glass formers(SiO₂+optionally B₂O₃, Al₂O₃) in relation to the poorly melting highlyrefractive components (MO (BaO, CaO+optionally SrO, MgO), TiO₂, Nb₂O₅,WO₃+optionally ZrO₂)

[0013] The glasses contain 27 to 40% by weight of the main glass formerSiO2. In the case of higher proportions, the desired high refractiveindex would not be reached and the fusibility would deteriorate; in thecase of lower proportions, the stability to crystallization and thechemical resistance would be reduced. An SiO₂ content of at least 29% byweight is preferred, particularly preferably of at least 31% by weightof SiO₂. A content of not more than 36% by weight is particularlypreferred.

[0014] For further stabilization, the glasses may contain up to 6% byweight of Al₂O₃, preferably up to <3% by weight of Al₂O₃, and up to<0.5% by weight of B₂O₃. Higher proportions of glass formers wouldreduce the fusibility. Preferably, Al₂O₃ is dispensed with. It is amajor advantage that the B₂O₃ content can remain limited to said lowproportions, since the aggressiveness of the glass melt is thus reduced,so that glasses containing extremely small amounts of Pt impurities andhence having very high transmittances can be produced in Pt components.

[0015] In order to achieve the desired optical position of a heavy flintglass, relatively high proportions of highly refractive components arerequired. The proportion of the glass formers and of fluxes having a lowrefractive index (Na₂O, K₂O) is therefore limited. Preferably 69.5% byweight of SiO₂+Al₂O₃+B₂O₃+Na₂O +K₂O are not exceeded, and veryparticularly preferably the limit of this sum is max. 60.5% by weight.

[0016] In addition to the glass formers, the glasses contain aproportion of fluxes which is sufficient for good fusibility. Thus, theycontain at least 8% by weight and not more than 28% by weight ofNa₂O+K₂O, and in particular 7-18% by weight of Na₂O and 1-10% by weightof K₂O. A flux content of 12-26% by weight with 9-16% by weight of Na₂Oand 3-10% by weight of K₂O is preferred, and at least 14% by weight ofNa₂O+K₂O with 10-15% by weight of Na₂O and 4-9% by weight of K₂O areparticularly preferred. A sum of Na₂O and K₂O of not more than 21% byweight is very particularly preferred.

[0017] The glasses contain the following highly refractive components:

[0018] They contain 1.5-<15% by weight of alkaline earth metal oxides,preferably 3.5-14, especially ≦11, particularly preferably ≦10, % byweight.

[0019] Specifically:

[0020] 1-10% by weight of BaO, preferably 3-10, particularly preferably3-8, % by weight

[0021] 0.5-5% by weight of CaO, preferably 0.5-3% by weight

[0022] 0-3% by weight of MgO, preferably 0-<2% by weight, preferablyMgo-free

[0023] 0-3% by weight of Sro, preferably 0-<2% by weight, preferablySrO-free

[0024] The proportion of alkaline earth metal oxide is limited to saidmaximum content since a further increase would be possible only byreducing the glass former and flux content and would lead tocrystallization effects, particularly since the further components whichincrease the refractive index are comparatively good nucleating agents.Said minimum contents of the alkaline earth metal oxides are necessaryin order to establish the high refractive index and to stabilize thechemical resistance.

[0025] The glasses contain 21-37% by weight of TiO_(2,) preferably23-35, particularly preferably 26-33, % by weight.

[0026] The glasses furthermore contain 5-17% by weight of Nb₂O₅,preferably >5, especially 7-15, particularly preferably ≦12, % byweight.

[0027] These two components form the basis of the high refractive indexat the desired Abbe number. An increase in the TiO₂ content would reducethe Abbe number excessively and also excessively increase the tendencyto crystallization. An increase in the Nb₂O₅ content would increase theAbbe number to a very excessive extent and slightly reduce therefractive index.

[0028] For stabilization to crystallization, the glasses may contain upto 7% by weight of ZrO_(2,) preferably <5% by weight. Preferably, theZrO₂ content replaces a corresponding part of the TiO₂ content, so thatpreferably the maximum sum of TiO₂+ZrO₂ is 37% by weight, in particular35% by weight.

[0029] A further increase of ZrO₂ would in turn lead to an increase inthe tendency to crystallization; furthermore the optical position wouldbe undesirably shifted.

[0030] By parallel use of different nucleating agents and crystalformers, namely TiO₂ in addition to Nb₂O₅ and optionally ZrO₂, theformation of defined crystals is impeded and it is possible to achievethe desired exceptional refractive index by means of high proportions ofthese components without adding lead.

[0031] An important component is WO₃. It is present in an amount of 0.1to 7% by weight in the glass and, in addition to the fine adjustment ofthe optical position, serves for further reducing the tendency tocrystallization by its spatial coordination, which is unusual in thisglass system. This too impedes the formation of defined crystals. Inthis glass system, a higher WO₃ content would in turn undesirably shiftthe optical position. A WO₃ content of between 0.2 and 5% by weight ispreferred, particularly preferably between 0.2 and 4% by weight.

[0032] In order to improve the glass quality, one or more refiningagents known per se can be added in the customary amount to the mixturefor refining the glass. Thus, the glass has a particularly good internalglass quality with respect to freedom from bubbles and freedom fromstria.

[0033] If, instead of As₂O₃, for example, Sb₂O₃ is used as refiningagent, preferably an amount of up to 1% by weight, which is possiblewithout losses in respect of the glass quality, the glasses which arelead-free according to the invention are additionally arsenic-free.

[0034] The glasses may also contain, for example, up to 1% by weight ofF⁻ and/or up to 1% by weight of Cl⁻. F⁻ is added, for example, as KF orKHF₂. Cl⁻ is added, for example, as NaCl.

[0035] The glasses from said composition range have refractive indicesn_(d) of between 1.65 and 1.80 and Abbe numbers ν_(d) of between 21 and33. Glasses from the composition ranges stated in each case as beingpreferred have refractive indices of n_(d) of between 1.68 and 1.79 andAbbe numbers ν_(d) of between 23 and 32. The refractive indices n_(d)and the Abbe numbers ν_(d) of glasses from the ranges stated as beingparticularly preferred are between 1.70 and 1.79 and between 24 and 28.

[0036] The entire disclosure of all applications, patents andpublications, cited herein and of corresponding German application No.10133763.9, filed Jul. 11, 2001 are incorporated by reference herein.

[0037] Without further elaboration, it is believed that one skilled inthe art can, using the preceding description, utilize the presentinvention to its fullest extent. The following preferred specificembodiments are, therefore, to be construed as merely illustrative, andnot limitative of the remainder of the disclosure in any way whatsoever.

[0038] In the foregoing and in the following examples, all temperaturesare set forth uncorrected in degrees Celsius and, all parts andpercentages are by weight, unless otherwise indicated.

EXAMPLES

[0039] Seven examples of glasses according to the invention wereproduced from customary raw materials by melting.

[0040] Table 2 shows the respective composition (in % by weight, basedon oxide), the refractive index n_(d), the Abbe number ν_(d), thepartial dispersion in the blue range of the spectrum P_(g,F) and theanomalous partial dispersion ΔP_(g,F), the density ρ[g/cm³], thecoefficient of thermal expansion α_(20/300) [10⁻⁶/K] and the glasstransition temperature T_(g) [°C] of the glasses.

[0041] By way of explanation:

[0042] The partial dispersive power, the so-called relative partialdispersion, in the blue part of the spectrum is represented by theexpression $P_{g,F} = {\frac{n_{g} - n_{F}}{n_{F} - n_{c}}.}$

[0043] By definition, the “normal line” obeys the equation

P_(g,F)=0.6438−0.001682·ν_(d)

[0044] in the blue range of the spectrum.

[0045] Glasses whose partial dispersion lies on this line are referredto as “normal glasses”.

[0046] In the case of glasses which have a dispersion behaviourdeviating from “normal glasses”, the ordinal difference ΔP_(g,F) bywhich the relevant P_(g,F)−ν_(d) point is shifted relative to the“normal line”, is stated:

[0047] A coarse classification of these glasses having anomalous partialdispersion into two groups is usual, depending on whether P_(g,F) is“above” (positive partial dispersion: ΔP_(g,F)=pos.) or “below”(negative partial dispersion: ΔP_(g,F)=neg.) the “normal line”.

[0048] The glasses according to the invention were produced as follows:the raw materials for the oxides, preferably carbonates and nitrates,were weighed out. The refining agent or agents was or were added, andthorough mixing was then carried out. The glass mixture was melted atabout 1350° C. in a continuous Pt melting unit, then refined at about1400° C. and thoroughly homogenized. At a pouring temperature of about1300° C., the glass was poured and was processed to give the desireddimensions.

[0049] Table 1 shows an example of melting. TABLE 1 Example of meltingfor 100 kg of glass (calculated) Oxide % by weight Raw material Weight[kg] SiO₂ 35.0 SiO₂ 34.97 Na₂O 9.0 Na₂CO₃ 15.39 K₂O 5.0 K₂CO₃ 7.34 CaO3.0 CaCO₃ 5.31 BaO 5.0 Ba(NO₃)₂ 0.34 BaCO₃ 6.18 TiO₂ 23.0 TiO₂ 23.11Nb₂O₃ 15.0 Nb₂O₅ 15.00 WO₃ 5.0 WO₃ 5.00 Sb₂O₃ 0.1 Sb₂O₃ 0.10

[0050] The properties of the glasses obtained are shown in table 2, inexample 3. TABLE 2 Glass compositions (in % by weight, based on oxide)and important properties: 1 2 3 4 5 6 7 SiO₂ 29.0 40.0 35.0 35.1 34.035.0 39.8 B₂O₃ 0.4 0.4 0.2 Al₂O₃ 6.0 2.5 6.0 Na₂O 16.0 9.0 9.0 12.0 11.113.0 12.3 K₂O 10.0 3.0 5.0 3.0 6.0 8.0 5.0 MgO 1.0 CaO 2.0 1.0 3.0 0.52.2 1.0 0.5 SrO 1.0 BaO 3.0 3.0 5.0 10.0 6.5 4.0 3.0 TiO₂ 23.0 34.0 23.028.0 29.5 29.0 23.0 ZrO₂ 3.0 Nb₂O₅ 7.0 7.0 15.0 10.0 9.5 7.0 7.0 WO₃ 2.03.0 5.0 1.0 0.7 0.5 0.2 Sb₂O₃ 0.1 0.1 0.1 0.1 0.1 0.1 0.1 n_(d) 1.68361.7816 1.7665 1.7700 1.7613 1.7251 1.6939 ν_(d) 31.26 23.82 26.49 26.1825.98 27.96 29.54 P_(g,F) 0.5964 0.6251 0.6128 0.6138 0.6126 0.60760.6056 ΔP_(g,F) (10⁻⁴) 52 214 135 140 131 109 115 ρ [g/cm³] 2.96 3.043.21 3.19 3.08 2.99 2.92 α_(20/300) [10⁻⁶/K] 13.5 8.4 9.5 10.0 10.5 11.39.8 Tg [° C.] 508 606 609 599 581 560 595

[0051] The glass composition range according to the invention offers agroup of lead-free and, in a preferred embodiment, also As₂O₃-freeoptical glasses of glass type HF having said optical data, which haveother properties improved compared with the known glasses. The freedomof the glasses from lead not only is advantageous because of theenvironmental protection concept discussed but also has a positiveeffect on their density and their glass transition temperature.

[0052] The glasses have the following advantages:

[0053] They have high chemical resistance; thus, they belong to the acidresistance class (ISO 8424) AR=2 or better and to the alkalineresistance class (ISO 10629) AR=2 or better, it being possible for therespective resistances to be 2.× or 1.×. The high chemical resistance ofthe glasses is important for their further processing, such as grindingand polishing.

[0054] The glasses have high stability to crystallization. It is thuspossible to produce the glasses in relatively large melting units, forexample in an optical trough.

[0055] The glasses are easy to melt and can be readily processed.

[0056] With at least 500° C., they have relatively high glass transitiontemperatures Tg.

[0057] Their density ρ of not more than 3.4 g/cm³ is very low. This isparticularly remarkable since the low density is not realized throughhigh B₂O₃ contents.

[0058] The transmittance of the glasses in the visible range of thespectrum is high. Thus, the spectral pure transmittance τ_(i) is at thewavelength λ=420 nm and a sample thickness of 25 mm (τ_(i 420) nm; 25mm>75%).

[0059] With these properties, especially with their optical position,their partial dispersion P_(g,F) and their anomalous partial dispersionΔP_(g,F) and their transmittance, the glasses are outstandingly suitablefor use as optical elements (lenses, prisms) as well as optical fibresand image-transmitting fibres in the optical applications of imaging,projection, telecommunication and laser technology.

[0060] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0061] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. Lead-free optical glasses having a refractive index n_(d) of between1.65 and 1.80 and an Abbe number ν_(d) of between 21 and 33,characterized by the following composition (in % by weight, based onoxide) SiO₂ 27-40 B₂O₃   0-<0.5 Al₂O₃ 0-6 Na₂O  7-18 K₂O  1-10 BaO  1-10SrO 0-3 CaO 0.5-5   MgO 0-3 with BaO + SrO + CaO + MgO <15 TiO₂ 21-37ZrO₂ 0-7 Nb₂O₅  5-17 WO₃ 0.1-7  

and optionally refining agents in the customary amounts.
 2. Lead-freeoptical glasses according to claim 1, having a refractive index n_(d) ofbetween 1.68 and 1.79 and an Abbe number ν_(d) of between 23 and 32,characterized by the following composition (in % by weight, based onoxide) SiO₂ 29-40 B₂O₃   0-<0.5 Al₂O₃  0-<3 Na₂O  9-16 K₂O  3-10 BaO 3-10 SrO  0-<2 CaO 0.5-3   MgO  0-<2 with BaO + SrO + CaO + MgO ≦14TiO₂ 23-35 ZrO₂  0-<5 Nb₂O₅  7-15 WO₃ 0.2-5  

and optionally refining agents in the customary amounts.
 3. Lead-freeoptical glasses according to claim 1 or 2, having a refractive indexn_(d) of between 1.70 and 1.79 and an Abbe number ν_(d) of between 24and 28, characterized by the following composition (in % by weight,based on oxide): SiO₂ 31-36 B₂O₃   0-<0.5 Na₂O 10-15 K₂O 4-9 BaO 3-8 CaO0.5-3   with BaO + CaO ≦10 TiO₂ 26-33 Nb₂O₅  7-12 WO₃ 0.2-4  

and optionally refining agents in the customary amounts.
 4. Lead-freeoptical glasses according to at least one of claims 1 to 3,characterized in that the glasses contain (% by weight): Sb₂O₃ 0-1 Cl⁻0-1 F⁻ 0-1


5. Lead-free optical glasses according to at least one of claims 1 to 4,characterized in that the glasses are free of arsenic oxide, with theexception of unavoidable impurities.
 6. Lead-free optical glassesaccording to at least one of claims 1 to 5, having a density ρ of notmore than 3.4 cm³ and a glass transition temperature T_(g) of ≧500° C.